1. Field of the Invention
The present invention relates to a wiring substrate and a manufacturing method thereof.
2. Description of the Related Art
In recent years, when producing a wiring substrate on which parts such as a semiconductor element, a chip, and so forth are to be mounted, the application of printing methods represented by an electrophotographic method, a screen printing, and the like is under study, and their practical uses are in progress (refer to, for example, Japanese Patent Laid-Open Application No. Hei7-263841 and Japanese Patent Laid-Open Application No. 2004-048030). For instance, a plating foundation layer is formed on an insulating substrate by printing by applying the electrophotographic method or the like, and electroless plating is performed to the plating foundation layer thereafter, so that a wiring layer is formed. The wiring layer forming step as described above allows the sheet-shaped substrate such as a resin film to be used as an insulating substrate, having an advantage that a flexible wiring substrate can be obtained easily.
In order to put the flexible wiring substrate into practical use, it is required to form wiring layers on both front and rear faces of the sheet-shaped insulating substrate and also to connect between the wiring layers on both the front and rear faces electrically. As methods that electrically connect between the front and rear faces of the substrate, generally, those connecting by a through hole and by a via hole are known in general. These connecting methods have problems that they are forced to have complicated steps to connect both the faces, for example: a foundation layer forming step forming a foundation layer along the internal wall surface of the through hole or via hole by the electroless plating, a resist forming step required before forming the foundation layer, and the like.
The flexible wiring substrates produced by applying the electrophotographic method or the like have an advantage of low cost based on a simplified production process, reduced number of steps, or the like. However, when the general through-hole connection or the via-hole connection is applied to the connection of both the front and rear faces of the substrate, the production cost of the flexible wiring substrate increases due to the complicated steps and increased number of steps required for the connection as described above. In other words, the flexible wiring substrate applying the electrophotographic method or the like cannot receive the benefit of low cost due to the step of connecting between both the front and rear faces of the substrate.
Meanwhile, as a simple method of connecting both the faces, there is known one that forms a through hole in the sheet-shaped substrate using a press method and prints a conductive ink on the through-hole portion from both the faces of the substrate to closely attach the conductive ink entered into the through hole from both sides thereon to thereby electrically connect the wiring layers provided on both the faces of the sheet-shaped substrate (refer to, for example, Japanese Patent Laid-Open Application No. Hei 2-246193). However, this method has a limitation in view of reliability improvement or resistance reduction of the connecting portion, and so forth, in that the inside of the through hole is filled based simply on fluidity, viscosity, and the like of the conductive ink.
In U.S. Pat. No. 5,049,244, there is disclosed a method in which a pinhole is formed in an insulating film, a metal thin film is formed on both front and rear faces of the insulating film and also on an internal wall surface of the pinhole by deposition method or sputtering, and thereafter wiring patterns on both the front and rear faces and a connecting portion connecting these are formed by plating using the metal thin film as a plating foundation layer. Due to the deposition method or sputtering applied to the formation of the plating foundation layer, this method cannot avoid a cost increase in connecting both the faces, and in addition to that, the reliability and characteristics of the both-face connecting portion are easily degraded depending on the shape of the pinhole. Further, as in the conventional through-hole forming steps, a resist-layer forming and removing steps are required, having a problem that the step of connecting both the faces itself becomes complicated.